Display device

ABSTRACT

A display device, an electronic device, or a lighting device that is unlikely to be broken is provided. A flexible first substrate and a flexible second substrate overlap with each other with a display element provided therebetween. A flexible third substrate is bonded on the outer surface of the first substrate, and a flexible fourth substrate is bonded on the outer surface of the second substrate. The third substrate is formed using a material softer than the first substrate, and the fourth substrate is formed using a material softer than the second substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to an object, a method,or a manufacturing method. In addition, one embodiment of the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. One embodiment of the present invention particularly relatesto a light-emitting device, a display device, an electronic device, alighting device, a manufacturing method thereof, a usage method thereof,an operation method thereof, or the like. In particular, the presentinvention relates to a light-emitting device, a display device, anelectronic device, or a lighting device utilizing electroluminescence(EL), a manufacturing method thereof, a usage method thereof, anoperation method thereof, or the like.

2. Description of the Related Art

Recent light-emitting devices and display devices are expected to beapplied to a variety of uses and become diversified.

For example, light-emitting devices and display devices for mobiledevices and the like are required to be thin, lightweight, capable ofbeing provided on a curved surface, and unlikely to be broken. Inaddition, a light-emitting device and a display device that can be bentat any part are demanded for greater portability.

Light-emitting elements utilizing EL (also referred to as EL elements)have features such as ease of thinning and lightening, high-speedresponse to input signal, and driving with a direct-current low voltagesource; therefore, application of the light-emitting elements tolight-emitting devices and display devices has been suggested.

For example, Patent Document 1 discloses a technical idea that a thinfilm device layer formed on a silicon wafer, a glass substrate, or thelike is transferred onto a plastic substrate having a stacked-layerstructure.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2004-72050

SUMMARY OF THE INVENTION

In Patent Document 1, materials of the plastic substrate that can beused for a display device are listed; however, some of them, such as afluorine rubber material or a silicone resin, are too soft to befavorable for transfer of a thin film device layer. Furthermore. PatentDocument 1 does not disclose a material for a substrate that can befavorably used in a repeatedly bendable display device.

An object of one embodiment of the present invention is to provide ahighly portable display device, electronic device, or lighting device.

Another object of one embodiment of the present invention is to providea repeatedly bendable display device, electronic device, or lightingdevice.

Another object of one embodiment of the present invention is to providea highly reliable display device, electronic device, or lighting device.

Another object of one embodiment of the present invention is to providea display device, electronic device, or lighting device that is unlikelyto be broken.

Another object of one embodiment of the present invention is to providea display device, electronic device, or lighting device with low powerconsumption.

Another object of one embodiment of the present invention is to providea novel display device, electronic device, or lighting device.

Note that the descriptions of these objects do not disturb the existenceof other objects. Note that in one embodiment of the present invention,there is no need to achieve all the objects. Note that other objectswill be apparent from the description of the specification, thedrawings, the claims, and the like and other objects can be derived fromthe description of the specification, the drawings, the claims, and thelike.

One embodiment of the present invention is a display device thatincludes a first substrate, a second substrate, a third substrate, and afourth substrate. The first substrate and the second substrate overlapwith each other with a display element provided therebetween. The thirdsubstrate and the fourth substrate overlap with each other with thefirst substrate and the second substrate provided therebetween. Thethird substrate and the fourth substrate are softer than the firstsubstrate and the second substrate.

One embodiment of the present invention is a display device thatincludes a first substrate, a second substrate, a third substrate, and afourth substrate. The first substrate and the second substrate overlapwith each other with a display element provided therebetween. The thirdsubstrate and the fourth substrate overlap with each other with thefirst substrate and the second substrate provided therebetween. TheYoung's modulus of the third substrate and the fourth substrate issmaller than the Young's modulus of the first substrate and the secondsubstrate.

The Young's modulus of a material suitable for the first substrate andthe second substrate is larger than or equal to 1 GPa (1×10⁹ Pa) andsmaller than or equal to 100 GPa (100×10⁹ Pa), preferably larger than orequal to 2 GPa and smaller than or equal to 50 GPa, further preferablylarger than or equal to 2 GPa and smaller than or equal to 20 GPa.

The Young's modulus of a material used for the third substrate and thefourth substrate is preferably smaller than or equal to one fiftieth,further preferably smaller than or equal to one hundredth, still furtherpreferably smaller than or equal to one five hundredth of the Young'smodulus of the material used for the first substrate and the secondsubstrate.

In one embodiment of the present invention, a highly portable displaydevice, electronic device, or lighting device can be provided.

In one embodiment of the present invention, a repeatedly bendabledisplay device, electronic device, or lighting device can be provided.

In one embodiment of the present invention, a highly reliable displaydevice, electronic device, or lighting device can be provided.

In one embodiment of the present invention, a display device, electronicdevice, or lighting device that is unlikely to be broken can beprovided.

In one embodiment of the present invention, a display device, electronicdevice, or lighting device with low power consumption can be provided.

In one embodiment of the present invention, a novel display device,electronic device, or lighting device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C illustrate one mode of a display device.

FIGS. 2A to 2C are cross-sectional views illustrating examples ofbending of a display device.

FIGS. 3A and 3B illustrate one mode of a display device.

FIGS. 4A to 4C are a block diagram and circuit diagrams illustrating onemode of a display device.

FIGS. 5A to 5C illustrate one mode of a display device.

FIGS. 6A to 6D are cross-sectional views illustrating one example of amethod for manufacturing a display device.

FIGS. 7A to 7D are cross-sectional views illustrating one example of amethod for manufacturing a display device.

FIGS. 8A and 8B are cross-sectional views illustrating one example of amethod for manufacturing a display device.

FIGS. 9A and 9B are cross-sectional views illustrating one example of amethod for manufacturing a display device.

FIGS. 10A and 10B are cross-sectional views illustrating one example ofa method for manufacturing a display device.

FIGS. 11A and 11B are cross-sectional views illustrating one example ofa method for manufacturing a display device.

FIG. 12 is a cross-sectional view illustrating one example of a methodfor manufacturing a display device.

FIGS. 13A to 13D are cross-sectional views illustrating one example of amethod for manufacturing a display device.

FIG. 14 is a cross-sectional view illustrating one mode of a displaydevice.

FIGS. 15A and 15B are cross-sectional views each illustrating one modeof a display device.

FIGS. 16A and 16B are cross-sectional views each illustrating one modeof a display device.

FIGS. 17A and 17B are cross-sectional views each illustrating one modeof a display device.

FIGS. 18A and 18B are cross-sectional views each illustrating one modeof a display device.

FIGS. 19A and 19B illustrate structure examples of light-emittingelements.

FIGS. 20A to 20E illustrate examples of electronic devices and lightingdevices.

FIGS. 21A and 21B illustrate one example of an electronic device.

FIGS. 22A to 22C illustrate one example of an electronic device.

FIGS. 23A and 23B are photographs for explaining Example.

FIGS. 24A to 24F each illustrate one mode of a display device.

FIG. 25 illustrates one mode of a display device.

FIGS. 26A to 26H each illustrate one mode of a display device.

FIGS. 27A and 27B are cross-sectional views each illustrating one modeof a display device.

FIGS. 28A and 28B are cross-sectional views each illustrating one modeof a display device.

FIGS. 29A and 29B are cross-sectional views each illustrating one modeof a display device.

FIG. 30 is a cross-sectional view illustrating one mode of a lightingdevice.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that one embodiment of the present invention is not limited to thefollowing description, and it will be easily understood by those skilledin the art that various changes and modifications can be made withoutdeparting from the spirit and scope of the present invention. Therefore,one embodiment of the present invention should not be construed as beinglimited to the description in the following embodiments. Note that inthe structures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and description of such portions is notrepeated.

Note that in each drawing referred to in this specification, the size ofeach component or the thickness of each layer might be exaggerated or aregion might be omitted for clarity of the invention. Therefore,embodiments of the invention are not limited to such scales. Especiallyin a top view (a plan view) and a perspective view, some componentsmight not be illustrated for easy understanding.

The position, size, range, and the like of each component illustrated inthe drawings and the like are not accurately represented in some casesto facilitate understanding of the invention. Therefore, the disclosedinvention is not necessarily limited to the position, size, range, andthe like disclosed in the drawings and the like. For example, in theactual manufacturing process, a resist mask or the like might beunintentionally reduced in size by treatment such as etching, which isnot illustrated in some cases for easy understanding.

Note that ordinal numbers such as “first” and “second” in thisspecification and the like are used in order to avoid confusion amongcomponents and do not denote the priority or the order such as the orderof steps or the stacking order. A term without an ordinal number in thisspecification and the like might be provided with an ordinal number in aclaim in order to avoid confusion among components.

In addition, in this specification and the like, the term such as an“electrode” or a “wiring” does not limit a function of a component. Forexample, an “electrode” is used as part of a “wiring” in some cases, andvice versa. Further, the term “electrode” or “wiring” can also mean acombination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

Note that the term “over” or “under” in this specification and the likedoes not necessarily mean that a component is placed “directly above andin contact with” or “directly below and in contact with” anothercomponent. For example, the expression “electrode B over insulatinglayer A” does not necessarily mean that the electrode B is on and indirect contact with the insulating layer A and can mean the case whereanother component is provided between the insulating layer A and theelectrode B.

Further, functions of a source and a drain might be switched dependingon operation condition, e.g., when a transistor having a differentpolarity is employed or a direction of current flow is changed incircuit operation. Therefore, it is difficult to define which is thesource (or the drain). Thus, the terms “source” and “drain” can be usedto denote the drain and the source, respectively.

In this specification and the like, the term “electrically connected”includes the case where components are connected through an objecthaving any electric function. There is no particular limitation on an“object having any electric function” as long as electric signals can betransmitted and received between components that are connected throughthe object. Thus, even when the expression “electrically connected” isused in this specification, there is a case in which no physicalconnection is made and a wiring is just extended in an actual circuit.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. In addition, the term “perpendicular” indicates that theangle formed between two straight lines is greater than or equal to 80°and less than or equal to 100°, and accordingly also includes the casewhere the angle is greater than or equal to 850 and less than or equalto 95°. The term “equal” allows for a maximum error of ±5%.

In this specification, in the case where an etching step is performedafter a photolithography process, a resist mask formed in thephotolithography process is removed after the etching step, unlessotherwise specified.

Embodiment 1

A structure example of a display device 100 that is one embodiment ofthe present invention will be described with reference to drawings. FIG.1A is a top view of the display device 100 and FIG. 1B is across-sectional view taken along a dashed-dotted line A1-A2 in FIG. 1A.FIG. 1C is a cross-sectional view taken along a dashed-dotted line B1-B2in FIG. 1A.

The cross-sectional structure of one embodiment of the present inventionis not limited to that illustrated in FIG. 1C. For example, any ofcross-sectional structures illustrated in FIGS. 24A to 24F may also beemployed. The external electrode 124 may be covered with a substrate 147as illustrated in FIGS. 24B, 24C, and 24F, in which case a connectionportion can be protected. Note that FIGS. 24D to 24F each illustrate astructure in which a semiconductor chip 910 is provided over a substrateby COG or the like. When the semiconductor chip 910 is covered with thesubstrate 147 as illustrated in FIGS. 24E and 24F, the semiconductorchip 910 and its connection portion can be protected.

FIGS. 2A to 2C are cross-sectional views illustrating the display device100 in a bent state. Note that FIGS. 2A to 2C are each a cross-sectionalview taken along the dashed-dotted line B1-B2 in FIG. 1A. FIG. 2Aillustrates the display device 100 which is folded double. FIG. 2Billustrates the display device 100 which is folded in three. FIG. 2Cillustrates the display device 100 which is rolled up. Note that thebending directions are not limited to those shown in FIGS. 2A to 2C, andthe display device 100 that is one embodiment of the present inventioncan be bent in any direction.

FIG. 3A is a perspective view of the display device 100, and FIG. 3B isa cross-sectional view for specifically describing a portion taken alonga dashed-dotted line X1-X2 in FIG. 3A. Note that the cross-sectionalstructure may be the one illustrated in FIG. 3B or the one illustratedin FIG. 25.

<Configuration Example of Display Device>

The display device 100 described in this embodiment includes a displayarea 131, a driver circuit 132, and a driver circuit 133. The displaydevice 100 also includes a terminal electrode 216 and a light-emittingelement 125 including an electrode 115, an EL layer 117, and anelectrode 118. A plurality of light-emitting elements 125 are formed inthe display area 131. A transistor 232 for controlling the amount oflight emitted from the light-emitting element 125 is connected to eachof the light-emitting elements 125.

The external electrode 124 and the terminal electrode 216 areelectrically connected to each other through an anisotropic conductiveconnection layer 123. In addition, the terminal electrode 216 iselectrically connected to the driver circuit 132 and the driver circuit133.

The driver circuit 132 and the driver circuit 133 each include aplurality of transistors 252. The driver circuit 132 and the drivercircuit 133 each have a function of determining which of thelight-emitting elements 125 in the display area 131 is supplied with asignal from the external electrode 124.

The transistor 232 and the transistor 252 each include a gate electrode206, a gate insulating layer 207, a semiconductor layer 208, a sourceelectrode 209 a, and a drain electrode 209 b. A wiring 219 is formed inthe same layer where the source electrode 209 a and the drain electrode209 b are formed. In addition, an insulating layer 210 is formed overthe transistor 232 and the transistor 252, and an insulating layer 211is formed over the insulating layer 210. The electrode 115 is formedover the insulating layer 211. The electrode 115 is electricallyconnected to the drain electrode 209 b through an opening formed in theinsulating layer 210 and the insulating layer 211. A partition 114 isformed over the electrode 115, and the EL layer 117 and the electrode118 are formed over the electrode 115 and the partition 114.

In the display device 100, a substrate 111 and a substrate 121 areattached to each other with a bonding layer 120 provided therebetween.One surface of the substrate 111 is provided with a substrate 137 with abonding layer 138 provided therebetween. One surface of the substrate121 is provided with the substrate 147 with a bonding layer 148 providedtherebetween.

The other surface of the substrate 111 is provided with an insulatinglayer 205 with a bonding layer 112 provided therebetween. The insulatinglayer 205 is preferably formed as a single layer or a multilayer usingsilicon oxide, silicon nitride, silicon oxynitride, silicon nitrideoxide, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, orthe like. The insulating layer 205 can be formed by a sputtering method,a CVD method, a thermal oxidation method, a coating method, a printingmethod, or the like.

The other surface of the substrate 121 is provided with an insulatinglayer 145 with a bonding layer 142 provided therebetween. The othersurface of the substrate 121 is provided with a light-blocking layer 264with the insulating layer 145 provided therebetween. The other surfaceof the substrate 121 is also provided with a coloring layer 266 and anovercoat layer 268 with the insulating layer 145 provided therebetween.

Note that the insulating layer 205 functions as a base layer and canprevent or reduce diffusion of moisture and impurity elements from thesubstrate 111, the bonding layer 112, or the like to the transistor orthe light-emitting element. The insulating layer 145 functions as a baselayer and can prevent or reduce diffusion of moisture and impurityelements from the substrate 121, the bonding layer 142, or the like tothe transistor or the light-emitting element. The insulating layer 145can be formed using a material and a method similar to those of theinsulating layer 205.

A flexible material such as an organic resin material, or the like canbe used for the substrate 111 and the substrate 121. In the case wherethe display device 100 is a so-called bottom-emission display device ora dual-emission display device, a material that transmits light emittedfrom the EL layer 117 is used for the substrate 111. In the case wherethe display device 100 is a top-emission display device or adual-emission display device, a material that transmits light emittedfrom the EL layer 117 is used for the substrate 121.

In a similar manner, in the case where the display device 100 is aso-called bottom-emission display device or a dual-emission displaydevice, a material that transmits light emitted from the EL layer 117 isused for the substrate 137. In the case where the display device 100 isa top-emission display device or a dual-emission display device, amaterial that transmits light emitted from the EL layer 117 is used forthe substrate 147.

If the mechanical strength of a material used for the substrate 111 andthe substrate 121 is too low, the substrates easily become deformed atthe time of manufacture of the display device 100, which reduces yieldand thus, contributes to a reduction in productivity. Yet, if themechanical strength of the material used for the substrate 111 and thesubstrate 121 is too high, the display device becomes difficult to bend.An index of the mechanical strength of a material is a Young's modulus.The Young's modulus of a material suitable for the substrate 111 and thesubstrate 121 is larger than or equal to 1 GPa (1×10⁹ Pa) and smallerthan or equal to 100 GPa (100×10⁹ Pa), preferably larger than or equalto 2 GPa and smaller than or equal to 50 GPa, further preferably largerthan or equal to 2 GPa and smaller than or equal to 20 GPa. Note that inmeasurement of a Young's modulus, ISO527, JISK7161, JISK7162. JISK7127,ASTMD638, ASTMD882, or the like can be referred to.

The thickness of each of the substrate 111 and the substrate 121 ispreferably greater than or equal to 5 μm and less than or equal to 100μm, further preferably greater than or equal to 10 μm and less than orequal to 50 μm. One or both of the substrate 111 and the substrate 121may be a stacked-layer substrate that includes a plurality of layers.

It is preferable that the substrate 111 and the substrate 121 be formedusing the same material and have the same thickness. However, dependingon the purpose, the substrates 111 and 121 may be formed using differentmaterials or have different thicknesses.

Examples of materials that have flexibility and transmit visible light,which can be used for the substrate 111 and the substrate 121, include apolyethylene terephthalate resin, a polyethylene naphthalate resin, apolyacrylonitrile resin, a polyimide resin, a polymethylmethacrylateresin, a polycarbonate resin, a polyethersulfone resin, a polyamideresin, a cycloolefin resin, a polystyrene resin, a polyamide imideresin, a polyvinylchloride resin, and the like. Furthermore, when alight-transmitting property is not necessary, a non-light-transmittingsubstrate may be used. For example, aluminum or the like may be used forthe substrate 121 or the substrate 111.

The thermal expansion coefficients of the substrate 111 and thesubstrate 121 are preferably less than or equal to 30 ppm/K, morepreferably less than or equal to 10 ppm/K. In addition, on surfaces ofthe substrate 111 and the substrate 121, a protective film having lowwater permeability may be formed in advance: examples of the protectivefilm include a film containing nitrogen and silicon such as a siliconnitride film or a silicon nitride oxide film and a film containingnitrogen and aluminum such as an aluminum nitride film. Note that astructure in which a fibrous body is impregnated with an organic resin(also called prepreg) may be used as the substrate 111 and the substrate121.

With such substrates, a non-breakable display device can be provided.Alternatively, a lightweight display device can be provided.Alternatively, an easily bendable display device can be provided.

For the substrate 137, a material softer than the substrate 111 is used.For example, a material having a smaller Young's modulus than thesubstrate 111 is used for the substrate 137. For the substrate 147, amaterial softer than the substrate 121 is used. For example, a materialhaving a smaller Young's modulus than the substrate 121 is used for thesubstrate 147.

The Young's modulus of the material used for the substrate 137 ispreferably smaller than or equal to one fiftieth, further preferablysmaller than or equal to one hundredth, still further preferably smallerthan or equal to one five hundredth of the Young's modulus of thematerial used for the substrate 111. The Young's modulus of the materialused for the substrate 147 is preferably smaller than or equal to onefiftieth, further preferably smaller than or equal to one hundredth,still further preferably smaller than or equal to one five hundredth ofthe Young's modulus of the material used for the substrate 121.

Examples of a material that can be used for the substrate 137 and thesubstrate 147 include a viscoelastic high molecular material such assilicone rubber or fluorine rubber. The material used for the substrate137 and the substrate 147 preferably has a light-transmitting property.The substrate 137 and the substrate 147 may be formed using the samekind of material or different materials.

In the case where the substrate 137 and the substrate 147 are bonded toeach other with the substrate 111 and the substrate 121 providedtherebetween, the thickness of the substrate 137 is preferably equal tothat of the substrate 147. When the thickness of the substrate 137 isequal to that of the substrate 147, the substrate 111 and the substrate121 can be positioned close to a neutral plane of a bent portion.Accordingly, stress applied to the substrate 111 and the substrate 121at the time of bending can be reduced.

A material with a small Young's modulus more easily becomes deformedthan a material with a large Young's modulus does; therefore, internalstress generated by deformation is easily dispersed in the former. Whena material with a Young's modulus smaller than that of the substrate 111and the substrate 121 is used for the substrate 137 and the substrate147, local stress generated in the substrate 111 and the substrate 121at the time of bending can be relaxed, whereby the substrate 111 and thesubstrate 121 can be prevented from being broken. The substrate 137 andthe substrate 147 also function as buffers dispersing external physicalpressure and impact.

The substrate 137 or the substrate 147 is provided on the inner side ofa bent portion, whereby the radius of curvature of the substrate 111 or121 that is positioned on the inner side of the bent portion can beprevented from being smaller than the thickness of the substrate 137 orthe substrate 147. In this manner, breakage of the substrate 111 or thesubstrate 121 due to bending at an excessively small radius of curvaturecan be prevented.

In one embodiment of the present invention, the display device 100 canbe prevented from being broken even when the radius of curvature of thesubstrate 111 or 121 that is positioned on the inner side of a bentportion is 1 mm or less.

Note that the thickness of the substrate 137 is preferably greater thanor equal to 2 times and less than or equal to 100 times that of thesubstrate 111, further preferably greater than or equal to 5 times andless than or equal to 50 times that of the substrate 111. The thicknessof the substrate 147 is preferably greater than or equal to 2 times andless than or equal to 100 times that of the substrate 121, furtherpreferably greater than or equal to 5 times and less than or equal to 50times that of the substrate 121. When the substrate 137 is thicker thanthe substrate 111 and the substrate 147 is thicker than the substrate121, stress relaxation and the effect of buffers can be enhanced.

It is preferable that the substrate 137 and the substrate 147 be formedusing the same material and have the same thickness. However, dependingon the purpose, the substrates 137 and 147 may be formed using differentmaterials or have different thicknesses.

Depending on the usage of the display device, it is also possible toprovide only one of the substrate 137 and the substrate 147. One or bothof the substrate 137 and the substrate 147 may be a stacked-layersubstrate that includes a plurality of layers.

In one embodiment of the present invention, a display device that isresistant to external impact and unlikely to be broken can be provided.

In one embodiment of the present invention, a highly reliable displaydevice can be provided which is unlikely to be broken even when it isrepeatedly bent and stretched.

<Example of Pixel Circuit Configuration>

Next, an example of a specific configuration of the display device 100is described with reference to FIGS. 4A to 4C. FIG. 4A is a blockdiagram illustrating the configuration of the display device 100. Thedisplay device 100 includes the display area 131, the driver circuit132, and the driver circuit 133. The driver circuit 132 functions as ascan line driver circuit, for example, and the driver circuit 133functions as a signal line driver circuit, for example.

The display device 100 includes m scan lines 135 which are arrangedparallel or substantially parallel to each other and whose potentialsare controlled by the driver circuit 132, and n signal lines 136 whichare arranged parallel or substantially parallel to each other and whosepotentials are controlled by the driver circuit 133. The display area131 includes a plurality of pixels 134 arranged in a matrix. The drivercircuit 132 and the driver circuit 133 are collectively referred to as adriver circuit portion in some cases.

Each of the scan lines 135 is electrically connected to the n pixels 134in the corresponding row among the pixels 134 arranged in m rows and ncolumns in the display area 131. Each of the signal lines 136 iselectrically connected to the m pixels 134 in the corresponding columnamong the pixels 134 arranged in m rows and n columns. Note that m and nare each an integer of 1 or more.

FIGS. 4B and 4C illustrate circuit configurations that can be used forthe pixels 134 in the display device illustrated in FIG. 4A.

[Example of Pixel Circuit for Light-Emitting Display Device]

The pixel 134 illustrated in FIG. 4B includes a transistor 431, acapacitor 233, the transistor 232, and the light-emitting element 125.

One of a source electrode and a drain electrode of the transistor 431 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 431 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 431 has a function of controlling whether to write a datasignal to a node 435 by being turned on or off.

One of a pair of electrodes of the capacitor 233 is electricallyconnected to the node 435, and the other is electrically connected to anode 437. The other of the source electrode and the drain electrode ofthe transistor 431 is electrically connected to the node 435.

The capacitor 233 functions as a storage capacitor for storing datawritten to the node 435.

One of a source electrode and a drain electrode of the transistor 232 iselectrically connected to a potential supply line VL_a, and the other iselectrically connected to the node 437. A gate electrode of thetransistor 232 is electrically connected to the node 435.

One of an anode and a cathode of the light-emitting element 125 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the node 437.

As the light-emitting element 125, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 125 is not limited toorganic EL elements, an inorganic EL element including an inorganicmaterial can be used.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel 134 in FIG. 4B, the pixels 134are sequentially selected row by row by the first driver circuit 132,whereby the transistors 431 are turned on and a data signal is writtento the nodes 435.

When the transistors 431 are turned off, the pixels 134 in which thedata has been written to the nodes 435 are brought into a holding state.Further, the amount of current flowing between the source electrode andthe drain electrode of the transistor 232 is controlled in accordancewith the potential of the data written to the node 435. Thelight-emitting element 125 emits light with a luminance corresponding tothe amount of flowing current. This operation is sequentially performedrow by row: thus, an image is displayed.

[Example of Pixel Circuit for Liquid Crystal Display Device]

The pixel 134 illustrated in FIG. 4C includes a liquid crystal element432, the transistor 431, and the capacitor 233.

The potential of one of a pair of electrodes of the liquid crystalelement 432 is set according to the specifications of the pixels 134 asappropriate. The alignment state of the liquid crystal element 432depends on data written to a node 436. A common potential may be appliedto one of the pair of electrodes of the liquid crystal element 432included in each of the plurality of pixels 134. Further, the potentialsupplied to one of a pair of electrodes of the liquid crystal element432 in the pixel 134 in one row may be different from the potentialsupplied to one of a pair of electrodes of the liquid crystal element432 in the pixel 134 in another row.

As examples of a driving method of the display device including theliquid crystal element 432, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell(ASM) mode, an optically compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA)mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode,and the like. Other examples of the driving method of the display deviceinclude an electrically controlled birefringence (ECB) mode, a polymerdispersed liquid crystal (PDLC) mode, a polymer network liquid crystal(PNLC) mode, and a guest-host mode. Note that the present invention isnot limited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

The liquid crystal element 432 may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. Liquid crystal exhibiting a blue phase does not needalignment treatment. In addition, the liquid crystal exhibiting a bluephase has a short response time of 1 msec or less and is opticallyisotropic, which makes the viewing angle dependence small.

Note that a display element other than the light-emitting element 125and the liquid crystal element 432 can be used. For example, anelectrophoretic element, an electronic ink, an electrowetting element, amicro electro mechanical system (MEMS), a digital micromirror device(DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), aninterferometric modulator (IMOD) element, or the like can be used as thedisplay element.

In the pixel 134 in the m-th row and the n-th column, one of a sourceelectrode and a drain electrode of the transistor 431 is electricallyconnected to a signal line DL_n, and the other is electrically connectedto the node 436. A gate electrode of the transistor 431 is electricallyconnected to a scan line GL_m. The transistor 431 has a function ofcontrolling whether to write a data signal to the node 436 by beingturned on or off.

One of a pair of electrodes of the capacitor 233 is electricallyconnected to a wiring to which a particular potential is supplied(hereinafter referred to as a capacitor line CL), and the other iselectrically connected to the node 436. The other of the pair ofelectrodes of the liquid crystal element 432 is electrically connectedto the node 436. The potential of the capacitor line CL is set inaccordance with the specifications of the pixel 134 as appropriate. Thecapacitor 233 functions as a storage capacitor for storing data writtento the node 436.

For example, in the display device including the pixel 134 in FIG. 4C,the pixels 134 are sequentially selected row by row by the first drivercircuit 132, whereby the transistors 431 are turned on and a data signalis written to the nodes 436.

When the transistors 431 are turned off, the pixels 134 in which thedata signal has been written to the nodes 436 are brought into a holdingstate. This operation is sequentially performed row by row; thus, animage is displayed.

<Modification Example>

FIGS. 5A to 5C illustrate a display device 200 having a structuredifferent from that of the display device 100. FIG. 5A is a top view ofthe display device 200 and FIG. 5B is a cross-sectional view taken alonga dashed-dotted line A3-A4 in FIG. 5A. FIG. 5C is a cross-sectional viewtaken along a dashed-dotted line B3-B4 in FIG. 5A.

The cross-sectional structure of one embodiment of the present inventionis not limited to that illustrated in FIG. 5C. For example, any ofcross-sectional structures illustrated in FIGS. 26A to 26H may also beemployed. The external electrode 124 may be covered with the substrate147 as illustrated in FIGS. 26B, 26C, 26D, 26G, and 26H, in which case aconnection portion can be protected. The external electrode 124 may becovered with the substrate 147 and the substrate 137 as illustrated inFIGS. 26D and 26H, in which case a connection portion can be protected.When the semiconductor chip 910 is covered with the substrate 147 asillustrated in FIGS. 26F, 26G, and 26H, the semiconductor chip 910 andits connection portion can be protected.

The display device 200 is different from the display device 100 in thatat least part of the substrate 137 and part of the substrate 147 extendbeyond the edges of the substrate 111 and the substrate 121 and that theextending portion of the substrate 137 and the extending portion of thesubstrate 147 are connected to each other. Other components can beformed in a manner similar to that of the display device 100. Note thatthe extending portions of the substrate 137 and the substrate 147 may beconnected directly or connected indirectly with a bonding layer or thelike provided therebetween.

The structure of the display device 200 can inhibit entry of impuritiesfrom the edges of the substrate 111 and the substrate 121 and thus canfurther improve the reliability of the display device.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Embodiment 2

In this embodiment, another example of a method for manufacturing thedisplay device 100 will be described with reference to FIGS. 6A to 6D,FIGS. 7A to 7D. FIGS. 8A and 8B, FIGS. 9A and 9B, FIGS. 10A and 10B,FIGS. 11A and 11B, and FIG. 12. Note that FIGS. 6A to 6D. FIGS. 7A to7D, FIGS. 8A and 8B, FIGS. 9A and 9B, FIGS. 10A and 10B, FIGS. 11A and11B, and FIG. 12 are each a cross-sectional view taken along adashed-dotted line X1-X2 in FIG. 3A.

<Example of Method for Manufacturing Display Device> [Formation ofSeparation Layer]

First, a separation layer 113 is formed over an element formationsubstrate 101 (see FIG. 6A). Note that the element formation substrate101 may be a glass substrate, a quartz substrate, a sapphire substrate,a ceramic substrate, a metal substrate, or the like. Alternatively, theelement formation substrate 101 may be a plastic substrate having heatresistance to the processing temperature in this embodiment.

As the glass substrate, for example, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used. Note that when the glass substrate contains a largeamount of barium oxide (BaO), the glass substrate can be heat-resistantand more practical. Alternatively, crystallized glass or the like may beused.

The separation layer 113 can be formed using an element selected fromtungsten, molybdenum, titanium, tantalum, niobium, nickel, cobalt,zirconium, ruthenium, rhodium, palladium, osmium, iridium, and silicon;an alloy material containing any of the elements; or a compound materialcontaining any of the elements. The separation layer 113 can also beformed to have a single-layer structure or a stacked-layer structureusing any of the materials. Note that the crystalline structure of theseparation layer 113 may be amorphous, microcrystalline, orpolycrystalline. The separation layer 113 can also be formed using ametal oxide such as aluminum oxide, gallium oxide, zinc oxide, titaniumdioxide, indium oxide, indium tin oxide, indium zinc oxide, or InGaZnO(IGZO).

The separation layer 113 can be formed by a sputtering method, a CVDmethod, a coating method, a printing method, or the like. Note that thecoating method includes a spin coating method, a droplet dischargemethod, and a dispensing method.

In the case where the separation layer 113 has a single-layer structure,the separation layer 113 is preferably formed using tungsten,molybdenum, or a tungsten-molybdenum alloy. Alternatively, theseparation layer 113 is preferably formed using an oxide or oxynitrideof tungsten, an oxide or oxynitride of molybdenum, or an oxide oroxynitride of a tungsten-molybdenum alloy.

In the case where the separation layer 113 has a stacked-layer structureincluding, for example, a layer containing tungsten and a layercontaining an oxide of tungsten, the layer containing an oxide oftungsten may be formed as follows: the layer containing tungsten isformed first and then an oxide insulating layer is formed in contacttherewith, so that the layer containing an oxide of tungsten is formedat the interface between the layer containing tungsten and the oxideinsulating layer. Alternatively, the layer containing an oxide oftungsten may be formed by performing thermal oxidation treatment, oxygenplasma treatment, treatment with a highly oxidizing solution such asozone water, or the like on the surface of the layer containingtungsten.

In this embodiment, a glass substrate is used as the element formationsubstrate 101. The separation layer 113 is formed of tungsten over theelement formation substrate 101 by a sputtering method.

[Formation of Insulating Layer]

Next, the insulating layer 205 is formed as a base layer over theseparation layer 113 (see FIG. 6A). The insulating layer 205 ispreferably formed as a single layer or a multilayer using silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum oxynitride, aluminum nitride oxide, or the like. Theinsulating layer 205 may have, for example, a two-layer structure ofsilicon oxide and silicon nitride or a five-layer structure in whichmaterials selected from the above are combined. The insulating layer 205can be formed by a sputtering method, a CVD method, a thermal oxidationmethod, a coating method, a printing method, or the like.

The thickness of the insulating layer 205 may be greater than or equalto 30 nm and less than or equal to 500 nm, preferably greater than orequal to 50 nm and less than or equal to 400 nm.

The insulating layer 205 can prevent or reduce diffusion of impurityelements from the element formation substrate 101, the separation layer113, or the like. Even after the element formation substrate 101 isreplaced by the substrate 111, the insulating layer 205 can prevent orreduce diffusion of impurity elements into the light-emitting element125 from the substrate 111, the bonding layer 112, or the like. In thisembodiment, the insulating layer 205 is formed by stacking a200-nm-thick silicon oxynitride film and a 50-nm-thick silicon nitrideoxide film by a plasma CVD method.

[Formation of Gate Electrode]

Next, the gate electrode 206 is formed over the insulating layer 205(see FIG. 6A). The gate electrode 206 can be formed using a metalelement selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten; an alloy containing any of these metalelements as a component; an alloy containing any of these metal elementsin combination; or the like. Further, one or more metal elementsselected from manganese and zirconium may be used. The gate electrode206 may have a single-layer structure or a stacked structure of two ormore layers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a two-layer structure in whicha copper film is stacked over a titanium film, a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order, and the like can be given. Alternatively, a film,an alloy film, or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

The gate electrode 206 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

First, a conductive film to be the gate electrode 206 is stacked overthe insulating layer 205 by a sputtering method, a CVD method, anevaporation method, or the like, and a resist mask is formed over theconductive film by a photolithography process. Next, part of theconductive film to be the gate electrode 206 is etched with the use ofthe resist mask to form the gate electrode 206. At the same time, awiring and another electrode can be formed.

The conductive film may be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Note thatin the case where the conductive film is etched by a dry etching method,ashing treatment may be performed before the resist mask is removed,whereby the resist mask can be easily removed using a stripper.

Note that the gate electrode 206 may be formed by an electrolyticplating method, a printing method, an inkjet method, or the like insteadof the above formation method.

The thickness of the gate electrode 206 is greater than or equal to 5 nmand less than or equal to 500 nm, preferably greater than or equal to 10nm and less than or equal to 300 nm, more preferably greater than orequal to 10 nm and less than or equal to 200 nm.

The gate electrode 206 may be formed using a light-blocking conductivematerial, whereby external light can be prevented from reaching thesemiconductor layer 208 from the gate electrode 206 side. As a result, avariation in electrical characteristics of the transistor due to lightirradiation can be suppressed.

[Formation of Gate Insulating Layer]

Next, the gate insulating layer 207 is formed (see FIG. 6A). The gateinsulating layer 207 can be formed to have a single-layer structure or astacked-layer structure using, for example, any of silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, a mixture of aluminum oxide and silicon oxide, hafnium oxide,gallium oxide, Ga—Zn-based metal oxide, and the like.

The gate insulating layer 207 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced. For example, a stackedlayer of silicon oxynitride and hafnium oxide may be used.

The thickness of the gate insulating layer 207 is preferably greaterthan or equal to 5 nm and less than or equal to 400 nm, furtherpreferably greater than or equal to 10 nm and less than or equal to 300nm, still further preferably greater than or equal to 50 nm and lessthan or equal to 250 nm.

The gate insulating layer 207 can be formed by a sputtering method, aCVD method, an evaporation method, or the like.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the gate insulating layer 207, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, andthe like can be given as examples.

The gate insulating layer 207 can have a stacked-layer structure inwhich a nitride insulating layer and an oxide insulating layer arestacked in this order from the gate electrode 206 side. When the nitrideinsulating layer is provided on the gate electrode 206 side, hydrogen,nitrogen, an alkali metal, an alkaline earth metal, or the like can beprevented from moving from the gate electrode 206 side to thesemiconductor layer 208. Note that nitrogen, an alkali metal, analkaline earth metal, or the like generally serves as an impurityelement of a semiconductor. In addition, hydrogen serves as an impurityelement of an oxide semiconductor. Thus, an “impurity” in thisspecification and the like includes hydrogen, nitrogen, an alkali metal,an alkaline earth metal, or the like.

In the case where an oxide semiconductor is used for the semiconductorlayer 208, the density of defect states at the interface between thegate insulating layer 207 and the semiconductor layer 208 can be reducedby providing the oxide insulating layer on the semiconductor layer 208side. Consequently, a transistor whose electrical characteristics arehardly degraded can be obtained. Note that in the case where an oxidesemiconductor is used for the semiconductor layer 208, an oxideinsulating layer containing oxygen in a proportion higher than that inthe stoichiometric composition is preferably formed as the oxideinsulating layer. This is because the density of defect states at theinterface between the gate insulating layer 207 and the semiconductorlayer 208 can be further reduced.

In the case where the gate insulating layer 207 is a stacked layer of anitride insulating layer and an oxide insulating layer as describedabove, it is preferable that the nitride insulating layer be thickerthan the oxide insulating layer.

The nitride insulating layer has a dielectric constant higher than thatof the oxide insulating layer, therefore, an electric field generatedfrom the gate electrode 206 can be efficiently transmitted to thesemiconductor layer 208 even when the gate insulating layer 207 has alarge thickness. When the gate insulating layer 207 has a large totalthickness, the withstand voltage of the gate insulating layer 207 can beincreased. Accordingly, the reliability of the semiconductor device canbe improved.

The gate insulating layer 207 can have a stacked-layer structure inwhich a first nitride insulating layer with few defects, a secondnitride insulating layer with a high blocking property against hydrogen,and an oxide insulating layer are stacked in that order from the gateelectrode 206 side. When the first nitride insulating layer with fewdefects is used in the gate insulating layer 207, the withstand voltageof the gate insulating layer 207 can be improved. Particularly when anoxide semiconductor is used for the semiconductor layer 208, the use ofthe second nitride insulating layer with a high blocking propertyagainst hydrogen in the gate insulating layer 207 makes it possible toprevent hydrogen contained in the gate electrode 206 and the firstnitride insulating layer from moving to the semiconductor layer 208.

An example of a method for forming the first and second nitrideinsulating layers is described below. First, a silicon nitride film withfew defects is formed as the first nitride insulating layer by a plasmaCVD method in which a mixed gas of silane, nitrogen, and ammonia is usedas a source gas. Next, a silicon nitride film in which the hydrogenconcentration is low and hydrogen can be blocked is formed as the secondnitride insulating layer by switching the source gas to a mixed gas ofsilane and nitrogen. By such a formation method, the gate insulatinglayer 207 in which nitride insulating layers with few defects and ablocking property against hydrogen are stacked can be formed.

The gate insulating layer 207 can have a stacked-layer structure inwhich a third nitride insulating layer with a high blocking propertyagainst an impurity, the first nitride insulating layer with fewdefects, the second nitride insulating layer with a high blockingproperty against hydrogen, and the oxide insulating layer are stacked inthat order from the gate electrode 206 side. When the third nitrideinsulating layer with a high blocking property against an impurity isprovided in the gate insulating layer 207, hydrogen, nitrogen, alkalimetal, alkaline earth metal, or the like, can be prevented from movingfrom the gate electrode 206 to the semiconductor layer 208.

An example of a method for forming the first to third nitride insulatinglayers is described below. First, a silicon nitride film with a highblocking property against an impurity is formed as the third nitrideinsulating layer by a plasma CVD method in which a mixed gas of silane,nitrogen, and ammonia is used as a source gas. Next, a silicon nitridefilm with few defects is formed as the first nitride insulating layer byincreasing the flow rate of ammonia. Then, a silicon nitride film inwhich the hydrogen concentration is low and hydrogen can be blocked isformed as the second nitride insulating layer by switching the sourcegas to a mixed gas of silane and nitrogen. By such a formation method,the gate insulating layer 207 in which nitride insulating layers withfew defects and a blocking property against an impurity are stacked canbe formed.

Moreover, in the case of forming a gallium oxide film as the gateinsulating layer 207, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Note that the threshold voltage of a transistor can be changed bystacking the semiconductor layer 208 in which a channel of thetransistor is formed and an insulating layer containing hafnium oxidewith an oxide insulating layer provided therebetween and injectingelectrons into the insulating layer containing hafnium oxide.

[Formation of Semiconductor Layer]

The semiconductor layer 208 can be formed using an amorphoussemiconductor, a microcrystalline semiconductor, a polycrystallinesemiconductor, or the like. For example, amorphous silicon ormicrocrystalline germanium can be used. Alternatively, a compoundsemiconductor such as silicon carbide, gallium arsenide, an oxidesemiconductor, or a nitride semiconductor, an organic semiconductor, orthe like can be used.

In the case of using an organic semiconductor for the semiconductorlayer 208, a low molecular organic material having an aromatic ring, an-electron conjugated conductive polymer, or the like can be used. Forexample, rubrene, tetracene, pentacene, perylenediimide,tetracyanoquinodimethane, polythiophene, polyacetylene, orpolyparaphenylene vinylene can be used.

In the case of using an oxide semiconductor for the semiconductor layer208, a c-axis aligned crystalline oxide semiconductor (CAAC-OS), apolycrystalline oxide semiconductor, a microcrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous oxide semiconductor, or the like can be used.

Note that an oxide semiconductor has an energy gap as wide as 3.0 eV ormore and high visible-light transmissivity. In a transistor obtained byprocessing an oxide semiconductor under appropriate conditions, theoff-state current at ambient temperature (e.g., 25° C.) can be less thanor equal to 100 zA (1×10⁻¹⁹ A), less than or equal to 10 zA (1×10⁻²⁰ A),and further less than or equal to 1 zA (1×10⁻²¹ A). Therefore, a displaydevice with low power consumption can be provided.

In the case where an oxide semiconductor is used for the semiconductorlayer 208, an insulating layer containing oxygen is preferably used asan insulating layer that is in contact with the semiconductor layer 208.

The thickness of the semiconductor layer 208 is greater than or equal to3 nm and less than or equal to 200 nm, preferably greater than or equalto 3 nm and less than or equal to 100 nm, more preferably greater thanor equal to 3 nm and less than or equal to 50 nm. In this embodiment, asthe semiconductor layer 208, an oxide semiconductor film with athickness of 30 nm is formed by a sputtering method.

Next, a resist mask is formed over the oxide semiconductor film, andpart of the oxide semiconductor film is selectively etched using theresist mask to form the semiconductor layer 208. The resist mask can beformed by a photolithography method, a printing method, an inkjetmethod, or the like as appropriate. Formation of the resist mask by aninkjet method needs no photomask; thus, fabrication cost can be reduced.

Note that the etching of the oxide semiconductor film may be performedby either one or both of a dry etching method and a wet etching method.After the etching of the oxide semiconductor film, the resist mask isremoved (see FIG. 6B).

[Formation of Source Electrode, Drain Electrode, and the Like]

Next, the source electrode 209 a, the drain electrode 209 b, the wiring219, and the terminal electrode 216 are formed (see FIG. 6C). First, aconductive film is formed over the gate insulating layer 207 and thesemiconductor layer 208.

The conductive film can have a single-layer structure or a stacked-layerstructure containing any of metals such as aluminum, titanium, chromium,nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, andtungsten or an alloy containing any of these metals as its maincomponent. For example, the following structures can be given: asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which an aluminum film is stacked over a titaniumfilm, a two-layer structure in which an aluminum film is stacked over atungsten film, a two-layer structure in which a copper film is stackedover a copper-magnesium-aluminum alloy film, a two-layer structure inwhich a copper film is stacked over a titanium film, a two-layerstructure in which a copper film is stacked over a tungsten film, athree-layer structure in which a titanium film or a titanium nitridefilm, an aluminum film or a copper film, and a titanium film or atitanium nitride film are stacked in this order, a three-layer structurein which a molybdenum film or a molybdenum nitride film, an aluminumfilm or a copper film, and a molybdenum film or a molybdenum nitridefilm are stacked in this order, and a three-layer structure in which atungsten film, a copper film, and a tungsten film are stacked in thisorder.

Note that a conductive material containing oxygen such as indium tinoxide, zinc oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added, or a conductive materialcontaining nitrogen such as titanium nitride or tantalum nitride may beused. It is also possible to use a stacked-layer structure formed usinga material containing the above metal element and conductive materialcontaining oxygen. It is also possible to use a stacked-layer structureformed using a material containing the above metal element and aconductive material containing nitrogen. It is also possible to use astacked-layer structure formed using a material containing the abovemetal element, a conductive material containing oxygen, and a conductivematerial containing nitrogen.

The thickness of the conductive film is greater than or equal to 5 nmand less than or equal to 500 nm, preferably greater than or equal to 10nm and less than or equal to 300 nm, more preferably greater than orequal to 10 nm and less than or equal to 200 nm. In this embodiment, a300-nm-thick tungsten film is formed as the conductive film.

Then, part of the conductive film is selectively etched using a resistmask to form the source electrode 209 a, the drain electrode 209 b, thewiring 219, and the terminal electrode 216 (including other electrodesand wirings formed using the same film). The resist mask can be formedby a photolithography method, a printing method, an inkjet method, orthe like as appropriate. Formation of the resist mask by an inkjetmethod needs no photomask; thus, fabrication cost can be reduced.

The conductive film may be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Note thatan exposed portion of the semiconductor layer 208 is removed by theetching step in some cases. After the etching of the conductive film,the resist mask is removed.

[Formation of Insulating Layer]

Next, the insulating layer 210 is formed over the source electrode 209a, the drain electrode 209 b, the wiring 219, and the terminal electrode216 (see FIG. 6D). The insulating layer 210 can be formed using amaterial and a method similar to those of the insulating layer 205.

In the case where an oxide semiconductor is used for the semiconductorlayer 208, an insulating layer containing oxygen is preferably used forat least part of the insulating layer 210 that is in contact with thesemiconductor layer 208. For example, in the case where the insulatinglayer 210 is a stack of a plurality of layers, at least a layer that isin contact with the semiconductor layer 208 is preferably formed usingsilicon oxide.

[Formation of Opening]

Next, part of the insulating layer 210 is selectively etched using aresist mask to form an opening 128 (see FIG. 6D). At the same time,another opening that is not illustrated is also formed. The resist maskcan be formed by a photolithography method, a printing method, an inkjetmethod, or the like as appropriate. Formation of the resist mask by aninkjet method needs no photomask; thus, fabrication cost can be reduced.

The insulating layer 210 may be etched by a dry etching method, a wetetching method, or both a dry etching method and a wet etching method.

The drain electrode 209 b and the terminal electrode 216 are partlyexposed by the formation of the opening 128. The resist mask is removedafter the formation of the opening 128.

[Formation of Planarization Film]

Next, the insulating layer 211 is formed over the insulating layer 210(see FIG. 7A). The insulating layer 211 can be formed using a materialand a method similar to those of the insulating layer 205.

Planarization treatment may be performed on the insulating layer 211 toreduce unevenness of a surface on which the light-emitting element 125is formed. The planarization treatment may be, but not particularlylimited to, polishing treatment (e.g. chemical mechanical polishing(CMP)) or dry etching treatment.

Forming the insulating layer 211 using an insulating material with aplanarization function can make polishing treatment unnecessary. As theinsulating material with a planarization function, for example, anorganic material such as a polyimide resin or an acrylic resin can beused. Besides such organic materials, a low-dielectric constant material(a low-k material) or the like can be used. Note that the insulatinglayer 211 may be formed by stacking a plurality of insulating layersformed of any of these materials.

Part of the insulating layer 211 that overlaps with the opening 128 isremoved to form an opening 129. At the same time, another opening thatis not illustrated is also formed. In addition, the insulating layer 211in a region to which the external electrode 124 is connected later isremoved. Note that the opening 129 or the like can be formed in such amanner that a resist mask is formed by a photolithography process overthe insulating layer 211 and a region of the insulating layer 211 thatis not covered with the resist mask is etched. A surface of the drainelectrode 209 b is exposed by the formation of the opening 129.

When the insulating layer 211 is formed using a photosensitive material,the opening 129 can be formed without the resist mask. In thisembodiment, a photosensitive polyimide resin is used to form theinsulating layer 211 and the opening 129.

[Formation of Anode]

Next, the electrode 115 is formed over the insulating layer 211 (seeFIG. 7B). The electrode 115 is preferably formed using a conductivematerial that efficiently reflects light emitted from the EL layer 117formed later. Note that the electrode 115 may have a stacked-layerstructure of a plurality of layers without limitation to a single-layerstructure. For example, in the case where the electrode 115 is used asan anode, a layer in contact with the EL layer 117 may be alight-transmitting layer, such as an indium tin oxide layer, having awork function higher than that of the EL layer 117, and a layer havinghigh reflectance (e.g., aluminum, an alloy containing aluminum, orsilver) may be provided in contact with the layer.

Note that although the display device having a top-emission structure isdescribed as an example in this embodiment, a display device having abottom-emission structure or a dual-emission structure may be used.

In the case where the display device 100 has a bottom-emission structureor a dual-emission structure, the electrode 115 is preferably formedusing a light-transmitting conductive material.

The electrode 115 can be formed in such a manner that a conductive filmto be the electrode 115 is formed over the insulating layer 211, aresist mask is formed over the conductive film, and a region of theconductive film that is not covered with the resist mask is etched. Theconductive film can be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Theresist mask can be formed by a photolithography method, a printingmethod, an inkjet method, or the like as appropriate. Formation of theresist mask by an inkjet method needs no photomask; thus, fabricationcost can be reduced. The resist mask is removed after the formation ofthe electrode 115.

[Formation of Partition]

Next, the partition 114 is formed (see FIG. 7C). The partition 114 isprovided in order to prevent an unintended electrical short-circuitbetween light-emitting elements 125 in adjacent pixels and unintendedlight emission from the light-emitting element 125. In the case of usinga metal mask for formation of the EL layer 117 described later, thepartition 114 has a function of preventing the contact of the metal maskwith the electrode 115. The partition 114 can be formed of an organicresin material such as an epoxy resin, an acrylic resin, or an imideresin or an inorganic material such as silicon oxide. The partition 114is preferably formed so that its sidewall has a tapered shape or atilted surface with a continuous curvature. The sidewall of thepartition 114 having the above-described shape enables favorablecoverage with the EL layer 117 and the electrode 118 formed later.

[Formation of EL Layer]

A structure of the EL layer 117 is described in Embodiment 6.

[Formation of Cathode]

The electrode 118 is used as a cathode in this embodiment, and thus ispreferably formed using a material that has a low work function and caninject electrons into the EL layer 117 described later. As well as asingle-layer of a metal having a low work function, a stack in which ametal material such as aluminum, a conductive oxide material such asindium tin oxide, or a semiconductor material is formed over aseveral-nanometer-thick buffer layer formed of an alkali metal or analkaline earth metal having a low work function may be used as theelectrode 118.

In the case where light emitted from the EL layer 117 is extractedthrough the electrode 118, the electrode 118 preferably has a propertyof transmitting visible light. The light-emitting element 125 includesthe electrode 115, the EL layer 117, and the electrode 118 (see FIG.7D).

[Formation of Counter Element Formation Substrate]

An element formation substrate 141 provided with the light-blockinglayer 264, the coloring layer 266, the overcoat layer 268, theinsulating layer 145, and a separation layer 143 is formed over theelement formation substrate 101 with the bonding layer 120 therebetween(see FIG. 8A). Note that the element formation substrate 141 is formedto face the element formation substrate 101 and may thus be called a“counter element formation substrate”. A structure of the elementformation substrate 141 (counter element formation substrate) isdescribed later.

The element formation substrate 141 is fixed over the element formationsubstrate 101 by the bonding layer 120. A light curable adhesive, areactive curable adhesive, a thermosetting adhesive, or an anaerobicadhesive can be used as the bonding layer 120. For example, an epoxyresin, an acrylic resin, or an imide resin can be used. In atop-emission structure, a drying agent (e.g., zeolite) having a sizeless than or equal to the wavelength of light or a filler (e.g.,titanium oxide or zirconium) with a high refractive index is preferablymixed into the bonding layer 120, in which case the efficiency ofextracting light emitted from the EL layer 117 can be improved.

[Separation of Element Formation Substrate from Insulating Layer]

Next, the element formation substrate 101 attached to the insulatinglayer 205 with the separation layer 113 therebetween is separated fromthe insulating layer 205 (see FIG. 8B). As a separation method,mechanical force (a separation process with a human hand or a gripper, aseparation process by rotation of a roller, ultrasonic waves, or thelike) may be used. For example, a cut is made in the separation layer113 with a sharp edged tool, by laser light irradiation, or the like andwater is injected into the cut. Alternatively, the cut is sprayed with amist of water. A portion between the separation layer 113 and theinsulating layer 205 absorbs water through capillarity action, so thatthe element formation substrate 101 can be separated easily from theinsulating layer 205.

[Bonding of Substrate]

Next, the substrate 111 is attached to the insulating layer 205 with thebonding layer 112 therebetween (see FIGS. 9A and 9B). The bonding layer112 can be formed using a material similar to that of the bonding layer120. In this embodiment, a 20-μm-thick aramid (polyamide resin) with aYoung's modulus of 10 GPa is used for the substrate 111.

[Separation of Counter Element Formation Substrate from InsulatingLayer]

Next, the element formation substrate 141 overlapping with theinsulating layer 145 with the separation layer 143 therebetween isseparated from the insulating layer 145 (see FIG. 10A). The elementformation substrate 141 can be separated in a manner similar to that ofthe above-described separation method of the element formation substrate101.

[Bonding of Substrate]

Next, the substrate 121 is attached to the insulating layer 145 with thebonding layer 142 therebetween (see FIG. 10B). The bonding layer 142 canbe formed using a material similar to that of the bonding layer 120. Thesubstrate 121 can be formed using a material similar to that of thesubstrate 111.

[Formation of Opening]

Next, the substrate 121, the bonding layer 142, the insulating layer145, the coloring layer 266, the overcoat layer 268, and the bondinglayer 120 in a region overlapping with the terminal electrode 216 andthe opening 128 are removed to form the opening 122 (see FIG. 11A). Asurface of the terminal electrode 216 is partly exposed by the formationof the opening 122.

[Formation of External Electrode]

Next, the anisotropic conductive connection layer 123 is formed in andon the opening 122, and the external electrode 124 for inputtingelectric power or a signal to the display device 100 is formed over theanisotropic conductive connection layer 123 (see FIG. 11B). The terminalelectrode 216 is electrically connected to the external electrode 124through the anisotropic conductive connection layer 123. For example, aflexible printed circuit (FPC) can be used as the external electrode124.

The anisotropic conductive connection layer 123 can be formed using anyof various anisotropic conductive films (ACF), anisotropic conductivepastes (ACP), and the like.

The anisotropic conductive connection layer 123 is formed by curing apaste-form or sheet-form material that is obtained by mixing conductiveparticles to a thermosetting resin or a thermosetting, light curableresin. The anisotropic conductive connection layer 123 exhibits ananisotropic conductive property by light irradiation orthermocompression bonding. As the conductive particles used for theanisotropic conductive connection layer 123, for example, particles of aspherical organic resin coated with a thin-film metal such as Au, Ni, orCo can be used.

[Bonding of Substrates]

Then, the substrate 137 is bonded to the substrate 111 with the bondinglayer 138 provided therebetween. The substrate 147 is bonded to thesubstrate 121 with the bonding layer 148 provided therebetween (see FIG.12). The bonding layers 138 and 148 can be formed using a materialsimilar to that of the bonding layer 120. In this embodiment, for thesubstrate 137 and the substrate 147, silicone rubber that has alight-transmitting property with respect to visible light, a thicknessof 200 μm, and a Young's modulus of 0.03 GPa is used.

[Components Formed Over Counter Element Formation Substrate]

Next, components, such as the light-blocking layer 264, formed over theelement formation substrate 141 are described with reference to FIGS.13A to 13D.

First, the element formation substrate 141 is prepared. The elementformation substrate 141 can be formed using a material similar to thatof the element formation substrate 101. Then, the separation layer 143and the insulating layer 145 are formed over the element formationsubstrate 141 (see FIG. 13A). The separation layer 143 can be formedusing a material and a method similar to those of the separation layer113. The insulating layer 145 can be formed using a material and amethod similar to those of the insulating layer 205.

Next, the light-blocking layer 264 is formed over the insulating layer145 (see FIG. 13B). After that, the coloring layer 266 is formed (seeFIG. 13C).

The light-blocking layer 264 and the coloring layer 266 each are formedin a desired position with any of various materials by a printingmethod, an inkjet method, a photolithography method, or the like.

Next, the overcoat layer 268 is formed over the light-blocking layer 264and the coloring layer 266 (see FIG. 13D).

For the overcoat layer 268, an organic insulating layer of an acrylicresin, an epoxy resin, polyimide, or the like can be used. With theovercoat layer 268, for example, an impurity or the like contained inthe coloring layer 266 can be prevented from diffusing into thelight-emitting element 125 side. Note that the overcoat layer 268 is notnecessarily formed.

A light-transmitting conductive film may be formed as the overcoat layer268. The light-transmitting conductive film is formed as the overcoatlayer 268, so that the light 235 emitted from the light-emitting element125 can be transmitted through the overcoat layer 268 and layersoverlapping with the overcoat layer 268, while ionized impurities can beprevented from passing through the overcoat layer 268.

The light-transmitting conductive film can be formed using, for example,indium oxide, indium tin oxide (ITO), indium zinc oxide, zinc oxide, orzinc oxide to which gallium is added. Graphene or a metal film that isthin enough to have a light-transmitting property can also be used.

Through the above-described steps, the components such as thelight-blocking layer 264 can be formed over the element formationsubstrate 141.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Embodiment 3

A display device 150 having a bottom-emission structure can befabricated by modification of the structure of the display device 100having a top-emission structure.

<Display Device Having Bottom-Emission Structure>

FIG. 14 illustrates an example of a cross-sectional structure of thedisplay device 150 having a bottom-emission structure. Note that FIG. 14is a cross-sectional view of a portion similar to the portion denoted bythe dashed-dotted line X1-X2 in FIG. 3A that is a perspective view ofthe display device 100. The display device 150 having a bottom-emissionstructure differs from the display device 100 in the position where thelight-blocking layer 264, the coloring layer 266, and the overcoat layer268 are formed. Specifically, in the display device 150, thelight-blocking layer 264, the coloring layer 266, and the overcoat layer268 are formed over the substrate 111.

In the display device 150, the substrate 121 on which the insulatinglayer 145 is directly formed can be attached to the substrate 111 withthe bonding layer 120 therebetween. In other words, the insulating layer145 does not need to be transferred from the element formation substrate141; thus, the element formation substrate 141, the separation layer143, and the bonding layer 142 are not needed. This can improve theproductivity, yield, and the like of the display device. Note that othercomponents of the display device 150 can be formed as in the case of thedisplay device 100.

In the display device 150 having a bottom-emission structure, theelectrode 115 is formed using a light-transmitting conductive material,and the electrode 118 is formed using a conductive material thatefficiently reflects light emitted from the EL layer 117.

In the display device 150, the light 235 emitted from the EL layer 117can be extracted from the substrate 111 side through the coloring layer266.

<Back Gate Electrode>

Note that the display device 150 is an example of a display device inwhich a transistor 272 is used as a transistor included in the drivercircuit 133. Although the transistor 272 can be formed in a mannersimilar to that of the transistor 252, the transistor 272 differs fromthe transistor 252 in that an electrode 263 is formed over theinsulating layer 210 in a region overlapping with the semiconductorlayer 208. The electrode 263 can be formed using a material and a methodsimilar to those of the gate electrode 206.

The electrode 263 can also serve as a gate electrode. In the case whereone of the gate electrode 206 and the electrode 263 is simply referredto as a “gate electrode”, the other may be referred to as a “back gateelectrode”. One of the gate electrode 206 and the electrode 263 may bereferred to as a “first gate electrode”, and the other may be referredto as a “second gate electrode”.

In general, the back gate electrode is formed using a conductive filmand positioned so that the channel formation region of the semiconductorlayer is positioned between the gate electrode and the back gateelectrode. Thus, the back gate electrode can function in a mannersimilar to that of the gate electrode. The potential of the back gateelectrode may be the same as that of the gate electrode or may be a GNDpotential or a predetermined potential. By changing a potential of theback gate electrode, the threshold voltage of the transistor can bechanged.

Furthermore, the gate electrode and the back gate electrode are formedusing conductive films and thus each have a function of preventing anelectric field generated outside the transistor from influencing thesemiconductor layer in which the channel is formed (in particular, afunction of blocking static electricity).

In the case where light is incident on the back gate electrode side,when the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Thus, photodegradation of thesemiconductor layer can be prevented and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

By providing the gate electrode 206 and the electrode 263 with thesemiconductor layer 208 therebetween and setting the potentials of thegate electrode 206 and the electrode 263 to be equal, a region of thesemiconductor layer 208 through which carriers flow is enlarged in thefilm thickness direction; thus, the number of transferred carriers isincreased. As a result, the on-state current and the field-effectmobility of the transistor are increased.

The gate electrode 206 and the electrode 263 each have a function ofblocking an external electric field; thus, charges in a layer under thegate electrode 206 and in a layer over the electrode 263 do not affectthe semiconductor layer 208. Thus, there is little change in thethreshold voltage in a stress test (e.g., a negative gate biastemperature (−GBT) stress test in which a negative voltage is applied toa gate or a +GBT stress test in which a positive voltage is applied to agate). In addition, changes in the rising voltages of on-state currentat different drain voltages can be suppressed.

The BT stress test is one kind of accelerated test and can evaluate, ina short time, change in characteristics (i.e., a change over time) oftransistors, which is caused by long-term use. In particular, the amountof change in threshold voltage of the transistor in the BT stress testis an important indicator when examining the reliability of thetransistor. As the amount of change in the threshold voltage in the BTstress test is small, the transistor has higher reliability.

By providing the gate electrode 206 and the electrode 263 and settingthe potentials of the gate electrode 206 and the electrode 263 to be thesame, the amount of change in the threshold voltage is reduced.Accordingly, variation in electrical characteristics among a pluralityof transistors is also reduced.

Note that a back gate electrode may be provided in the transistor 232formed in the display area 131.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Embodiment 4

A display device 160 in which the coloring layer 266, the light-blockinglayer 264, the overcoat layer 268, and the like are not provided can bemanufactured by modification of the structure of the display device 100having a top-emission structure.

FIG. 15A illustrates an example of a cross-sectional structure of thedisplay device 160. Note that FIGS. 15A and 15B are cross-sectionalviews of a portion similar to the portion denoted by the dashed-dottedline X1-X2 in FIG. 3A that is a perspective view of the display device100. In the display device 160, color display can be performed by usingan EL layer 117A, an EL layer 117B, an EL layer 117C (not shown), andthe like instead of the light-blocking layer 264, the coloring layer266, and the overcoat layer 268. The EL layer 117A, the EL layer 117B,and the like can emit light of the respective colors such as red, blue,and green. For example, the EL layer 117A emits light 235A of a redwavelength, the EL layer 117B emits light 235B of a blue wavelength, andthe EL layer 117C emits light 235C (not shown) of a green wavelength.

Since the coloring layer 266 is not provided, a reduction in luminancecaused when the light 235A, light 235B, and light 235C are transmittedthrough the coloring layer 266 can be prevented. The thicknesses of theEL layer 117A, EL layer 117B, and EL layer 117C are adjusted inaccordance with the wavelengths of the light 235A, light 235B, and light235C, whereby a higher color purity can be achieved.

Note that in a manner similar to that of the display device 160, adisplay device 170 in which the coloring layer 266, the light-blockinglayer 264, the overcoat layer 268, and the like are not provided canalso be manufactured by modification of the structure of the displaydevice 150 having a bottom emission structure. FIG. 15B illustrates anexample of a cross-sectional structure of the display device 170.

Note that as illustrated in FIGS. 27A and 27B, an optical film 911,examples of which include a polarizing plate, a retardation plate, and aquarter-wave plate, may be additionally provided. The optical film 911is bonded with the use of a bonding layer 148A or a bonding layer 138A.This structure can reduce reflection at a screen surface. Moreover, theoptical film 911 can be protected.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Embodiment 5

In the display device 100, a substrate provided with a touch sensor maybe provided over the substrate 147 as illustrated in FIG. 16A. The touchsensor is formed using a conductive layer 991, a conductive layer 993,and the like. In addition, an insulating layer 992 is formed between theconductive layers.

As the conductive layer 991 and/or the conductive layer 993, atransparent conductive film of indium tin oxide, indium zinc oxide, orthe like is preferably used. Note that a layer containing alow-resistance material may be used for part or the whole of theconductive layer 991 and/or the conductive layer 993 in order to reduceresistance. For example, the conductive layer 991 and/or the conductivelayer 993 can be formed as a single layer or a stack using any of metalssuch as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten and an alloycontaining any of these metals as a main component. Alternatively, ametal nanowire may be used as the conductive layer 991 and/or theconductive layer 993. Silver or the like is preferably used as a metalfor the metal nanowire, in which case the resistance value can bereduced and the sensitivity of the sensor can be improved.

The insulating layer 992 is preferably formed as a single layer or amultilayer using silicon oxide, silicon nitride, silicon oxynitride,silicon nitride oxide, aluminum oxide, aluminum oxynitride, aluminumnitride oxide, or the like. The insulating layer 992 can be formed by asputtering method, a CVD method, a thermal oxidation method, a coatingmethod, a printing method, or the like.

Although the touch sensor is provided over the substrate 994, oneembodiment of the present invention is not limited thereto. The touchsensor may be provided under the substrate 994 (i.e., between thesubstrate 121 and the substrate 994).

The substrate provided with the touch sensor may be positioned under thesubstrate 137 in the display device 150. FIG. 16B illustrates an exampleof this case.

The touch sensor may be positioned over the substrate 121 with thebonding layer 148A provided therebetween as illustrated in FIG. 28A, ormay be positioned under the substrate 111 with the bonding layer 138Aprovided therebetween as illustrated in FIG. 28B.

Note that the substrate 994 may have a function of an optical film. Thatis, the substrate 994 may have a function of a polarizing plate, aretardation plate, or the like.

In the display device 100, the substrate 121 may be provided with atouch sensor. FIG. 17A illustrates an example in which the substrate 121is provided with a touch sensor and the substrate 147 is formed over thetouch sensor and the bonding layer 142.

In the display device 150, the substrate 111 may be provided with atouch sensor. FIG. 17B illustrates an example in which the substrate 111is provided with a touch sensor and the substrate 137 is formed underthe touch sensor and the bonding layer 138.

In the display device 160, the substrate 121 may be provided with atouch sensor. FIG. 18A illustrates an example in which the substrate 121is provided with a touch sensor and the substrate 147 is formed over thetouch sensor and the bonding layer 142.

In the display device 170, the substrate 111 may be provided with atouch sensor. FIG. 18B illustrates an example in which the substrate 111is provided with a touch sensor and the substrate 137 is formed underthe touch sensor and the bonding layer 138.

Note that in FIGS. 18A and 18B, the optical film 911 may be provided.FIGS. 29A and 29B illustrate an example of this case. The optical film911 is bonded with the use of a bonding layer 142A or the bonding layer138A.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Embodiment 6

In this embodiment, structure examples of a light-emitting element thatcan be applied to the light-emitting element 125 are described. Notethat an EL layer 320 described in this embodiment corresponds to the ELlayer 117 described in the above embodiment.

<Structure of Light-Emitting Element>

In a light-emitting element 330 illustrated in FIG. 19A, the EL layer320 is interposed between a pair of electrodes (an electrode 318 and anelectrode 322). Note that the electrode 318 is used as an anode and theelectrode 322 is used as a cathode as an example in the followingdescription of this embodiment.

The EL layer 320 includes at least a light-emitting layer and may have astacked-layer structure including a functional layer other than thelight-emitting layer. As the functional layer other than thelight-emitting layer, a layer containing a substance having a highhole-injection property, a substance having a high hole-transportproperty, a substance having a high electron-transport property, asubstance having a high electron-injection property, a bipolar substance(a substance having high electron- and hole-transport properties), orthe like can be used. Specifically, functional layers such as ahole-injection layer, a hole-transport layer, an electron-transportlayer, and an electron-injection layer can be used in combination asappropriate.

The light-emitting element 330 illustrated in FIG. 19A emits light whencurrent flows because of a potential difference generated between theelectrode 318 and the electrode 322 and holes and electrons arerecombined in the EL layer 320. That is, the light-emitting region isformed in the EL layer 320.

In the present invention, light emitted from the light-emitting element330 is extracted to the outside from the electrode 318 side or theelectrode 322 side. Therefore, one of the electrode 318 and theelectrode 322 is formed of a light-transmitting substance.

Note that a plurality of EL layers 320 may be stacked between theelectrode 318 and the electrode 322 as in a light-emitting element 331illustrated in FIG. 19B. In the case where n (n is a natural number of 2or more) layers are stacked, a charge generation layer 320 a ispreferably provided between an m-th EL layer 320 and an (m+1)-th ELlayer 320. Note that m is a natural number greater than or equal to 1and less than n.

The charge generation layer 320 a can be formed using a compositematerial of an organic compound and a metal oxide, a metal oxide, acomposite material of an organic compound and an alkali metal, analkaline earth metal, or a compound thereof; alternatively, thesematerials can be combined as appropriate. Examples of the compositematerial of an organic compound and a metal oxide include compositematerials of an organic compound and a metal oxide such as vanadiumoxide, molybdenum oxide, and tungsten oxide. As the organic compound, avariety of compounds can be used; for example, low molecular compoundssuch as an aromatic amine compound, a carbazole derivative, and aromatichydrocarbon and oligomers, dendrimers, and polymers of these lowmolecular compounds. As the organic compound, it is preferable to usethe organic compound which has a hole-transport property and has a holemobility of 10⁻⁶ cm²/Vs or higher. However, substances other than thesubstances given above may also be used as long as the substances havehole-transport properties higher than electron-transport properties.These materials used for the charge generation layer 320 a haveexcellent carrier-injection properties and carrier-transport properties;thus, the light-emitting element 330 can be driven with low current andwith low voltage.

Note that the charge generation layer 320 a may be formed with acombination of a composite material of an organic compound and a metaloxide with another material. For example, a layer containing a compositematerial of the organic compound and the metal oxide may be combinedwith a layer containing a compound of a substance selected fromsubstances with an electron-donating property and a compound with a highelectron-transport property. Moreover, a layer containing a compositematerial of the organic compound and the metal oxide may be combinedwith a transparent conductive film.

The light-emitting element 331 having such a structure is unlikely tosuffer the problem of energy transfer, quenching, or the like and has anexpanded choice of materials, and thus can easily have both highemission efficiency and a long lifetime. Moreover, it is easy to obtainphosphorescence from one light-emitting layer and fluorescence from theother light-emitting layer.

The charge generation layer 320 a has a function of injecting holes toone of the EL layers 320 that is in contact with the charge generationlayer 320 a and a function of injecting electrons to the other EL layer320 that is in contact with the charge generation layer 320 a, whenvoltage is applied between the electrode 318 and the electrode 322.

The light-emitting element 331 illustrated in FIG. 19B can provide avariety of emission colors by changing the type of the light-emittingsubstance used for the EL layer 320. In addition, a plurality oflight-emitting substances emitting light of different colors may be usedas the light-emitting substances, whereby light emission having a broadspectrum or white light emission can be obtained.

In the case of obtaining white light emission using the light-emittingelement 331 illustrated in FIG. 19B, as for the combination of aplurality of EL layers, a structure for emitting white light includingred light, green light, and blue light may be used: for example, thestructure may include a light-emitting layer containing a bluefluorescent substance as a light-emitting substance and a light-emittinglayer containing red and green phosphorescent substances aslight-emitting substances. Alternatively, a structure including alight-emitting layer emitting red light, a light-emitting layer emittinggreen light, and a light-emitting layer emitting blue light may beemployed. Further alternatively, with a structure includinglight-emitting layers emitting light of complementary colors, whitelight emission can be obtained. In a stacked-layer element including twolight-emitting layers in which light emitted from one of thelight-emitting layers and light emitted from the other light-emittinglayer have complementary colors to each other, the combinations ofcolors are as follows: blue and yellow, blue-green and red, and thelike.

Note that in the structure of the above-described stacked-layer element,by providing the charge generation layer between the stackedlight-emitting layers, the element can have a long lifetime in ahigh-luminance region while keeping the current density low. Inaddition, the voltage drop due to the resistance of the electrodematerial can be reduced, whereby uniform light emission in a large areais possible.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Embodiment 7

In this embodiment, examples of an electronic device and a lightingdevice including the display device of one embodiment of the presentinvention are described with reference to drawings.

As examples of the electronic devices including a flexible displaydevice, the following can be given: television devices (also calledtelevisions or television receivers), monitors of computers or the like,digital cameras, digital video cameras, digital photo frames, mobilephones (also called cellular phones or mobile phone devices), portablegame machines, portable information terminals, audio reproducingdevices, large game machines such as pachinko machines, and the like.

In addition, a lighting device or a display device can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

FIG. 20A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using the display device in the display portion 7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 20A is touched with a finger or the like, data can be input to themobile phone 7400. Further, operations such as making a call andinputting a character can be performed by touch on the display portion7402 with a finger or the like.

The power can be turned on or off with the operation button 7403. Inaddition, types of images displayed on the display portion 7402 can beswitched; for example, switching images from a mail creation screen to amain menu screen is performed with the operation button 7403.

Here, the display portion 7402 includes the display device of oneembodiment of the present invention. Thus, the mobile phone can have acurved display portion and high reliability.

FIG. 20B illustrates an example of a wristband-type display device. Aportable display device 7100 includes a housing 7101, a display portion7102, an operation button 7103, and a sending and receiving device 7104.

The portable display device 7100 can receive a video signal with thesending and receiving device 7104 and can display the received video onthe display portion 7102. In addition, with the sending and receivingdevice 7104, the portable display device 7100 can send an audio signalto another receiving device.

With the operation button 7103, power ON/OFF, switching displayedvideos, adjusting volume, and the like can be performed.

Here, the display portion 7102 includes the display device of oneembodiment of the present invention. Thus, the portable display devicecan have a curved display portion and high reliability.

FIGS. 20C to 20E illustrate examples of lighting devices. Lightingdevices 7200, 7210, and 7220 each include a stage 7201 provided with anoperation switch 7203 and a light-emitting portion supported by thestage 7201.

The lighting device 7200 illustrated in FIG. 20C includes alight-emitting portion 7202 with a wave-shaped light-emitting surfaceand thus is a good-design lighting device.

A light-emitting portion 7212 included in the lighting device 7210illustrated in FIG. 20D has two convex-curved light-emitting portionssymmetrically placed. Thus, light radiates from the lighting device7210.

The lighting device 7220 illustrated in FIG. 20E includes aconcave-curved light-emitting portion 7222. This is suitable forilluminating a specific range because light emitted from thelight-emitting portion 7222 is collected to the front of the lightingdevice 7220.

The light-emitting portion included in each of the lighting devices7200, 7210, and 7220 is flexible; thus, the light-emitting portion maybe fixed on a plastic member, a movable frame, or the like so that anemission surface of the light-emitting portion can be bent freelydepending on the intended use.

The light-emitting portions included in the lighting devices 7200, 7210,and 7220 each include the display device of one embodiment of thepresent invention. Thus, the lighting devices whose display portions canbe curved into any shape and which has high reliability can be provided.

FIG. 30 shows a cross-sectional view of a lighting device.

FIG. 21A illustrates an example of a portable display device. A displaydevice 7300 includes a housing 7301, a display portion 7302, operationbuttons 7303, a display portion pull 7304, and a control portion 7305.

The display device 7300 includes a rolled flexible display portion 7302in the cylindrical housing 7301.

The display device 7300 can receive a video signal with the controlportion 7305 and can display the received video on the display portion7302. In addition, a battery is included in the control portion 7305.Moreover, a connector may be included in the control portion 7305 sothat a video signal or power can be supplied directly.

With the operation buttons 7303, power ON/OFF, switching of displayedvideos, and the like can be performed.

FIG. 21B illustrates a state in which the display portion 7302 is pulledout with the display portion pull 7304. Videos can be displayed on thedisplay portion 7302 in this state. Further, the operation buttons 7303on the surface of the housing 7301 allow one-handed operation.

Note that a reinforcement frame may be provided for an edge portion ofthe display portion 7302 in order to prevent the display portion 7302from being curved when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with a video signal.

The display portion 7302 includes the display device of one embodimentof the present invention. Thus, the display portion 7302 is a flexible,highly reliable display device, which makes the display device 7300lightweight and highly reliable.

FIGS. 22A and 22B illustrate a double foldable tablet terminal 9600 asan example. FIG. 22A illustrates the tablet terminal 9600 which isunfolded. The tablet terminal 9600 includes a housing 9630, a displayportion 9631, a display mode switch 9626, a power switch 9627, apower-saving mode switch 9625, a clasp 9629, and an operation switch9628.

The housing 9630 includes a housing 9630 a and a housing 9630 b, whichare connected with a hinge portion 9639. The hinge portion 9639 makesthe housing 9630 double foldable.

The display portion 9631 is provided on the housing 9630 a, the housing9630 b, and the hinge portion 9639. By the use of the display devicethat is disclosed in the present specification and the like, the tabletterminal in which the display portion 9631 can be bent and which hashigh reliability can be provided.

Part of the display portion 9631 can be a touchscreen region 9632 anddata can be input when a displayed operation key 9638 is touched. Astructure can be employed in which half of the display portion 9631 hasonly a display function and the other half has a touchscreen function.The whole display portion 9631 may have a touchscreen function. Forexample, keyboard buttons may be displayed on the entire region of thedisplay portion 9631 so that the display portion 9631 can be used as adata input terminal.

The display mode switch 9626 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power-saving switch 9625 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. Another detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal, in addition to the opticalsensor.

FIG. 22B illustrates the tablet terminal 9600 which is folded. Thetablet terminal 9600 includes the housing 9630, a solar cell 9633, and acharge and discharge control circuit 9634. As an example, FIG. 22Billustrates the charge and discharge control circuit 9634 including thebattery 9635 and the DC-to-DC converter 9636.

By including the display device that is disclosed in the presentspecification and the like, the display portion 9631 is foldable. Sincethe tablet terminal 9600 is double foldable, the housing 9630 can beclosed when the tablet terminal is not in use, for example; thus, thetablet terminal is highly portable. Moreover, since the display portion9631 can be protected when the housing 9630 is closed, the tabletterminal can have high durability and high reliability for long-termuse.

The tablet terminal illustrated in FIGS. 22A and 22B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touchscreen, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 ispreferably provided on both surfaces of the housing 9630, in which casethe battery 9635 can be charged efficiently. When a lithium ion batteryis used as the battery 9635, there is an advantage of downsizing or thelike.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 22B is described with reference to a blockdiagram of FIG. 22C. FIG. 22C illustrates the solar cell 9633, thebattery 9635, the DC-to-DC converter 9636, a converter 9637, switchesSW1 to SW3, and the display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 22B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 with the use of externallight. The voltage of the power generated by the solar cell is raised orlowered by the DC-to-DC converter 9636 so as to be voltage for chargingthe battery 9635. Then, when power from the solar cell 9633 is used forthe operation of the display portion 9631, the switch SW1 is turned onand the voltage of the power is raised or lowered by the converter 9637so as to be voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration unit, the power generation unit is not particularly limited,and the battery 9635 may be charged by another power generation unitsuch as a piezoelectric element or a thermoelectric conversion element(Peltier element). For example, the battery 9635 may be charged by anon-contact power transmission module which is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charge unit used in combination.

It is needless to say that one embodiment of the present invention isnot limited to the above-described electronic devices and lightingdevices as long as the display device of one embodiment of the presentinvention is included.

This embodiment can be implemented in an appropriate combination withany of the structures described in other embodiments.

Example

A display device 500 was formed by the method described in Embodiment 2.As a display element, a light-emitting element including an organic ELmaterial was used. Note that for the substrate 111 and the substrate121, a 20-μm-thick aramid with a Young's modulus of approximately 10 GPawas used. For the substrate 137 and the substrate 147, 200-μm-thicksilicone rubber with a Young's modulus of approximately 0.03 GPa wasused.

Then, the entire display region of the display device 500 was made toemit light and the display device 500 was folded double such that thedisplay region on the left of the bent portion and that on the right ofthe bent portion overlapped with each other. FIG. 23A is a photographshowing the display device 500 folded double in an emission state.

Then, the display device 500 folded double was unfolded, and a displaystate of the display device 500 was observed. FIG. 23B is a photographshowing the display device 500 that was unfolded. Even after the displaydevice 500 was folded and unfolded, the entire display region thereofemitted light and the display state remained to be excellent.

Note that a display device which was not provided with the substrate 137and the substrate 147 was fabricated as a comparative sample by themethod described in Embodiment 2, and folded double. In this displaydevice, the substrate 111 and the substrate 121 were cracked, and lightemission was not maintained in the entire display region.

This application is based on Japanese Patent Application serial no.2013-169542 filed with Japan Patent Office on Aug. 19, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a first substrate and a second substrate overlapping with each other with a display element provided therebetween; and a third substrate and a fourth substrate overlapping with each other with the first substrate and the second substrate provided therebetween, wherein the first substrate and the second substrate are in contact with the third substrate and the fourth substrate, respectively, wherein a Young's modulus of the first substrate and the second substrate is larger than or equal to 1 GPa and smaller than or equal to 100 GPa, wherein a Young's modulus of the third substrate and the fourth substrate is smaller than or equal to one fiftieth of the Young's modulus of the first substrate and the second substrate, and wherein the display device is configured to be bent at any part.
 2. The display device according to claim 1, wherein the display element is a light-emitting element.
 3. The display device according to claim 2, wherein the light-emitting element is electrically connected to a transistor.
 4. The display device according to claim 3, wherein a semiconductor layer of the transistor comprises an oxide semiconductor.
 5. The display device according to claim 1, wherein at least one of a first pair of the first substrate and the third substrate and a second pair of the second substrate and the fourth substrate has a light-transmitting property.
 6. The display device according to claim 1, wherein a thickness of each of the third substrate and the fourth substrate is greater than or equal to 2 times and less than or equal to 100 times that of the first substrate or the second substrate.
 7. A display device comprising: a first substrate and a second substrate overlapping with each other with a display element provided therebetween; and a third substrate and a fourth substrate overlapping with each other with the first substrate and the second substrate provided therebetween, wherein the first substrate and the second substrate are in contact with the third substrate and the fourth substrate, respectively, wherein a Young's modulus of the first substrate and the second substrate is larger than or equal to 1 GPa and smaller than or equal to 100 GPa, wherein a Young's modulus of the third substrate and the fourth substrate is smaller than or equal to one fiftieth of the Young's modulus of the first substrate and the second substrate, wherein the third substrate and the fourth substrate comprise a viscoelastic high molecular material, and wherein the display device is configured to be bent at any part.
 8. The display device according to claim 7, wherein the display element is a light-emitting element.
 9. The display device according to claim 8, wherein the light-emitting element is electrically connected to a transistor.
 10. The display device according to claim 9, wherein a semiconductor layer of the transistor comprises an oxide semiconductor.
 11. The display device according to claim 7, wherein at least one of a first pair of the first substrate and the third substrate and a second pair of the second substrate and the fourth substrate has a light-transmitting property.
 12. The display device according to claim 7, wherein a thickness of each of the third substrate and the fourth substrate is greater than or equal to 2 times and less than or equal to 100 times that of the first substrate or the second substrate.
 13. A display device comprising: a first substrate and a second substrate overlapping with each other with a display element provided therebetween; and a third substrate and a fourth substrate overlapping with each other with the first substrate and the second substrate provided therebetween, wherein the first substrate and the second substrate are in contact with the third substrate and the fourth substrate, respectively, wherein a Young's modulus of the first substrate and the second substrate is larger than or equal to 1 GPa and smaller than or equal to 100 GPa, wherein a Young's modulus of the third substrate and the fourth substrate is smaller than or equal to one fiftieth of the Young's modulus of the first substrate and the second substrate, wherein the third substrate and the fourth substrate comprise silicone rubber or fluorine rubber, and wherein the display device is configured to be bent at any part.
 14. The display device according to claim 13, wherein the display element is a light-emitting element.
 15. The display device according to claim 14, wherein the light-emitting element is electrically connected to a transistor.
 16. The display device according to claim 15, wherein a semiconductor layer of the transistor comprises an oxide semiconductor.
 17. The display device according to claim 13, wherein at least one of a first pair of the first substrate and the third substrate and a second pair of the second substrate and the fourth substrate has a light-transmitting property.
 18. The display device according to claim 13, wherein a thickness of each of the third substrate and the fourth substrate is greater than or equal to 2 times and less than or equal to 100 times that of the first substrate or the second substrate. 